Restrictive reuse set management

ABSTRACT

A restrictive reuse set management algorithm for equal grade of service on a forward link (FL) transmission is described. While a restrictive reuse algorithm is known to improve the SINR of weak users in interference limited networks, the fairness issue associated with restrictive reuse has not been explored. The bandwidth allocated to each reuse set, the reuse set loading and the harmonic mean of the spectral efficiencies of all users sharing the reuse set are used to estimate the equal-grade-of-service (EGoS) user throughput over the reuse set. An iterative algorithm is then designed to maximize the sector throughput under the constraint of equal user throughput over all reuse sets.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/590,549, entitled, “ASBR Reuse Set Management Approach forEqual Grade of Service on FL Transmission,” filed Jul. 23, 2004, andassigned to the assignee hereof and hereby expressly incorporated byreference herein.

REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT

The present Application for Patent is related to the followingco-pending U.S. patent application: “Feedback to Support RestrictiveReuse” Ser. No. 11/020,707 filed Dec. 22, 2004, assigned to the assigneehereof, and expressly incorporated by reference herein.

The present Application for Patent is related to the followingco-pending U.S. patent application: “Restrictive Reuse for a WirelessCommunication System” Ser. No. 10/871,084 filed Jun. 18, 2004, assignedto the assignee hereof, and expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention relates generally to communication, morespecifically to data transmission in a wireless multiple-accesscommunication system.

2. Background

A wireless multiple-access system can concurrently support communicationfor multiple wireless terminals on the forward and reverse links. Theforward link (or downlink) refers to the communication link from basestations to terminals, and the reverse link (or uplink) refers to thecommunication link from terminals to base stations. Multiple terminalsmay simultaneously transmit data on the reverse link and/or receive dataon the forward link. This may be achieved by multiplexing the datatransmissions on each link to be orthogonal to one another in time,frequency, and/or code domain. The orthogonality ensures that the datatransmission for each terminal does not interfere with the datatransmissions for other terminals.

A multiple-access system typically has many cells, where the term “cell”can refer to a base station and/or its coverage area depending on thecontext in which the term is used. Data transmissions for terminals inthe same cell may be sent using orthogonal multiplexing to avoid“intra-cell” interference. However, data transmissions for terminals indifferent cells may not be orthogonalized, in which case each terminalwould observe “inter-cell” interference from other cells. The inter-cellinterference may significantly degrade performance for certaindisadvantaged terminals observing high levels of interference.

To combat inter-cell interference, a wireless system may employ afrequency reuse scheme whereby not all frequency bands available in thesystem are used in each cell. For example, a system may employ a 7-cellreuse pattern and a reuse factor of K=7. For this system, the overallsystem bandwidth W is divided into seven equal frequency bands, and eachcell in a 7-cell cluster is assigned one of the seven frequency bands.Each cell uses only one frequency band, and every seventh cell reusesthe same frequency band. With this frequency reuse scheme, the samefrequency band is only reused in cells that are not adjacent to eachother, and the inter-cell interference observed in each cell is reducedrelative to the case in which all cells use the same frequency band.However, a large reuse factor represents inefficient use of theavailable system resources since each cell is able to use only afraction of the overall system bandwidth.

There is therefore a need in the art for techniques to reduce inter-cellinterference in a more efficient manner.

SUMMARY

In an aspect, a method of allocating system resources in a wirelesscommunication system, comprises determining dimension of each reuse set,determining loading of each reuse set, determining spectral efficienciesof all users sharing each reuse set, estimating anequal-grade-of-service (EGoS) user throughput over each reuse set basedon the dimension of each reuse set, loading of each reuse set, andspectral efficiencies of all users sharing each reuse set, andequalizing user throughputs in all reuse sets. In an aspect, the methodfurther comprises maximizing sector throughput under the constraint ofEGoS user throughput over each reuse set. In an aspect, the methodfurther comprises scheduling a user from each reuse set.

In an aspect, user throughputs in all reuse sets are equalized byiteratively moving users from low throughput reuse sets to highthroughput reuse sets. In an aspect, two reuse sets swap users if thethroughputs in both reuse sets are increased.

In an aspect, the dimension is a subcarrier in an OFDMA system. Inanother aspect, the dimension is a timeslot-subcarrier in an OFDMAsystem. In yet another aspect, the dimension is a timeslot in a TDMAsystem. In yet another aspect, the dimension is a carrier in an MC-CDMAsystem.

In an aspect, an apparatus operable to allocate system resources in awireless communication system comprises a controller operative todetermining dimension of each reuse set, determining loading of eachreuse set, determining spectral efficiencies of all users sharing eachreuse set, estimating an equal-grade-of-service (EGoS) user throughputover each reuse set based on the dimension of each reuse set, loading ofeach reuse set, and spectral efficiencies of all users sharing eachreuse set, and equalizing user throughputs in all reuse sets.

In another aspect, an apparatus operable to allocate system resources ina wireless communication system comprises means for determiningdimension of each reuse set, means for determining loading of each reuseset, means for determining spectral efficiencies of all users sharingeach reuse set, means for estimating an equal-grade-of-service (EGoS)user throughput over each reuse set based on the dimension of each reuseset, loading of each reuse set, and spectral efficiencies of all userssharing each reuse set, and means for equalizing user throughputs in allreuse sets.

In an aspect, a computer readable media embodying a method forallocating system resources in a wireless communication system comprisesdetermining dimension of each reuse set, determining loading of eachreuse set, determining spectral efficiencies of all users sharing eachreuse set, estimating an equal-grade-of-service (EGoS) user throughputover each reuse set based on the dimension of each reuse set, loading ofeach reuse set, and spectral efficiencies of all users sharing eachreuse set; and equalizing user throughputs in all reuse sets.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a wireless multiple-access communication system;

FIGS. 2A and 2B show a sectorized cell and its model, respectively;

FIG. 3 shows an exemplary multi-cell layout with 3-sector cells;

FIG. 4 shows three overlapping forbidden sets for three sectors;

FIGS. 5A through 5D show four unrestricted and restricted sets for asector;

FIG. 6 shows an example for forming three forbidden subband sets;

FIGS. 7A through 7D show a distribution of four users in a cluster ofseven sectors and non-interference patterns for three of the users;

FIG. 8 shows a process for allocating subbands to users with restrictivereuse;

FIG. 9 shows a block diagram of a transmitting entity;

FIG. 10 shows a block diagram of a receiving entity;

FIG. 11 shows a flowchart of a reuse set management approach inaccordance with an embodiment, and

FIG. 12 shows the average SINR (geometry) of an OFDMA system with andwithout restrictive reuse set management approach in accordance with anembodiment.

DETAILED DESCRIPTION

Techniques to efficiently avoid or reduce interference from stronginterferers in a wireless communication system are described herein. Astrong interferer to a given user u may be a base station (on theforward link) or another user (on the reverse link). User u may also bea strong interferer to other users. A strong interference entity foruser u may be a strong interferer causing high interference to user uand/or a strong interferes observing high interference from or due touser u. Strong interference entities (or interferers/interferees, orsimply, interferers/ees) for each user may be identified as describedbelow. Users are allocated system resources (e.g., frequency subbands)that are orthogonal to those used by their strong interferers/ees andthus avoid interfering with one another. These techniques are called“restrictive reuse” techniques and may be used for various wirelesssystems and for both the forward and reverse links.

In an embodiment of restrictive reuse, each cell/sector is assigned (1)a set of usable subbands that may be allocated to users in thecell/sector and (2) a set of forbidden subbands that are not allocatedto the users in the cell/sector. The usable set and the forbidden setfor each cell/sector are orthogonal to one other. The usable set foreach cell/sector also overlaps the forbidden set for each neighboringcell/sector. A given user u in a cell/sector x may be allocated subbandsin the usable set for that cell/sector. If user u observes (or causes)high level of interference from (to) a neighboring cell/sector y, thenuser u may be allocated subbands from a “restricted” set that containssubbands included in both the usable set for cell/sector x and theforbidden set for cell/sector y.

User u would then observe (cause) no interference from (to) cell/sectory since the subbands allocated to user u are members of the forbiddenset not used by cell/sector y. The subband restriction may be extendedto avoid interference from multiple neighboring cells/sectors.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a wireless multiple-access communication system 100. System100 includes a number of base stations 110 that support communicationfor a number of wireless terminals 120. A base station is a fixedstation used for communicating with the terminals and may also bereferred to as an access point, a Node B, or some other terminology.Terminals 120 are typically dispersed throughout the system, and eachterminal may be fixed or mobile. A terminal may also be referred to as amobile station, a user equipment (UE), a wireless communication device,or some other terminology. Each terminal may communicate with one orpossibly multiple base stations on the forward and reverse links at anygiven moment.

For a centralized architecture, a system controller 130 couples to thebase stations and provides coordination and control for these basestations. For a distributed architecture, the base stations maycommunicate with one another as needed, e.g., to serve a terminal,coordinate usage of system resources, and so on.

FIG. 2A shows a cell 210 with three sectors. Each base station providescommunication coverage for a respective geographic area. The coveragearea of each base station may be of any size and shape and is typicallydependent on various factors such as terrain, obstructions, and so on.To increase capacity, the base station coverage area may be partitionedinto three sectors 212 a, 212 b, and 212 c, which are labeled as sectors1, 2, and 3, respectively. Each sector may be defined by a respectiveantenna beam pattern, and the three beam patterns for the three sectorsmay point 120° from each other. The size and shape of each sector aregenerally dependent on the antenna beam pattern for that sector, and thesectors of the cell typically overlap at the edges. A cell/sector maynot be a contiguous region, and the cell/sector edge may be quitecomplex.

FIG. 2B shows a simple model for sectorized cell 210. Each of the threesectors in cell 210 is modeled by an ideal hexagon that approximates theboundary of the sector. The coverage area of each base station may berepresented by a clover of three ideal hexagons centered at the basestation.

Each sector is typically served by a base transceiver subsystem (BTS).In general, the term “sector” can refer to a BTS and/or its coveragearea, depending on the context in which the term is used. For asectorized cell, the base station for that cell typically includes theBTSs for all sectors of that cell. For simplicity, in the followingdescription, the term “base station” is used generically for both afixed station that serves a cell and a fixed station that serves asector. A “serving” base station or “serving” sector is one with which aterminal communicates. The terms “terminal” and “user” are also usedinterchangeably herein.

The restrictive reuse techniques may be used for various communicationsystems such as TDMA, FDMA, OFDMA and Multi-Carrier CDMA systems. Themain difference between different systems are the specific orthogonaldimensions, e.g., time slot in TDMA systems, frequency in FDMA systems,subcarrier/subbands in OFDMA systems, and carriers in Multi-Carrier CDMAsystems.

For clarity, these techniques are described for an Orthogonal FrequencyDivision Multiple Access (OFDMA) system that utilizes orthogonalfrequency division multiplexing (OFDM). OFDM effectively partitions theoverall system bandwidth into a number of (N) orthogonal frequencysubbands, which are also referred to as tones, sub-carriers, bins,frequency channels, and so on. Each subband is associated with arespective sub-carrier that may be modulated with data.

In the OFDMA system, multiple orthogonal “traffic” channels may bedefined whereby (1) each subband is used for only one traffic channel inany given time interval and (2) each traffic channel may be assignedzero, one, or multiple subbands in each time interval. A traffic channelmay be viewed as a convenient way of expressing an assignment ofsubbands for different time intervals. Each terminal may be assigned adifferent traffic channel. For each sector, multiple data transmissionsmay be sent simultaneously on multiple traffic channels withoutinterfering with one another.

The OFDMA system may or may not use frequency hopping (FH). Withfrequency hopping, a data transmission hops from subband to subband in apseudo-random manner, which can provide frequency diversity and otherbenefits. For a frequency hopping OFDMA (FH-OFDMA) system, each trafficchannel may be associated with a specific FH sequence that indicates theparticular subband(s) to use for that traffic channel in each timeinterval (or hop period). The FH sequences for different trafficchannels in each sector are orthogonal to one another so that no twotraffic channels use the same subband in any given hop period. The FHsequences for each sector may also be pseudo-random with respect to theFH sequences for neighboring sectors. These properties for the FHsequences minimize intra-sector interference and randomize inter-sectorinterference.

In the OFDMA system, users with different channel conditions may bedistributed throughout the system. These users may have differentcontribution and tolerance to inter-sector interference. The channelcondition for each user may be quantified by a signal quality metric,which may be defined by a signal-to-interference-and-noise ratio (SINR),a channel gain, a received pilot power, and/or some other quantitymeasured for the user's serving base station, some other measurements,or any combination thereof. A weak user has a relatively poor signalquality metric (e.g., a low SINR) for its serving base station, e.g.,due to a low channel gain for its serving base station and/or highinter-sector interference. A weak user may in general be locatedanywhere within a sector but is typically located far away from theserving base station. In general, a weak user is less tolerant tointer-sector interference, causes more interference to users in othersectors, has poor performance, and may be a bottleneck in a system thatimposes a fairness requirement.

Restrictive reuse can avoid or reduce interference observed/caused byweak users. This may be achieved by determining the likely sources ofhigh inter-sector interference (or strong interferers) and/or the likelyvictims of high inter-sector interference (or strong interferees) forthe weak users. The strong interferers may be base stations (on theforward link) and/or users (on the reverse link) in neighboring sectors.The strong interferees may be users in neighboring sectors. In any case,the weak users are allocated subbands that are orthogonal to those usedby the strong interferers/ees.

In an embodiment of restrictive reuse, each sector x is assigned ausable subband set (denoted as Ux) and a forbidden or unused subband set(denoted as Fx). The usable set contains subbands that may be allocatedto the users in the sector. The forbidden set contains subbands that arenot allocated to users in the sector. The usable set and the forbiddenset for each sector are orthogonal or disjoint in that no subband isincluded in both sets. The usable set for each sector also overlaps theforbidden set for each neighboring sector. The forbidden sets formultiple neighboring sectors may also overlap. The users in each sectormay be allocated subbands from the usable set as described below.

Restrictive reuse may be used for systems composed of unsectorized cellsas well as systems composed of sectorized cells. For clarity,restrictive reuse is described below for an exemplary system composed of3-sector cells.

FIG. 3 shows an exemplary multi-cell layout 300 with each 3-sector cellbeing modeled by a clover of three hexagons. For this cell layout, eachsector is surrounded in the first tier (or the first ring) by sectorsthat are labeled differently from that sector. Thus, each sector 1 issurrounded by six sectors 2 and 3 in the first tier, each sector 2 issurrounded by six sectors 1 and 3, and each sector 3 is surrounded bysix sectors 1 and 2.

FIG. 4 shows a Venn diagram illustrating a formation of threeoverlapping sets of subbands, labeled as F1, F2 and F3, which may beused as three forbidden subband sets. In this example, each forbiddenset overlaps with each of the other two forbidden sets (e.g., forbiddenset F1 overlaps with each of forbidden sets F2 and F3). Because of theoverlapping, an intersection set operation on any two forbidden setsyields a non-empty set. This property may be expressed as follows:F₁₂=F₁∩F₂≠Θ, F₁₃=F₁∩F₃≠Θ, and F₂₃=F₂∩F₃≠Θ,  Eq (1)

-   where “∩” denotes an intersection set operation;-   F_(xy) is a set containing subbands that are members of both sets    F_(x) and F_(y); and-   Θ denotes a null/empty set.

Each of the three forbidden sets F1, F2 and F3 is a subset of a full setΩ that contains all N total subbands, or F₁⊂Ω, F₂⊂Ω, and F₃⊂Ω. Forefficient utilization of the available subbands, the three forbiddensets may also be defined such that there is no overlap over all threesets, which may be expressed as:F₁₂₃=F₁∩F₂∩F₃=Θ.  Eq (2)

The condition in equation (2) ensures that each subband is used by atleast one sector.

Three usable subband sets U1, U2 and U3 may be formed based on the threeforbidden subband sets F1, F2 and F3, respectively. Each usable set Uxmay be formed by a difference set operation between the full set Ω andforbidden set Fx, as follows:U₁=Ω\F₁, U₂=Ω\F₂, and U₃=Ω\F₃,  (3)

-   where “\” denotes a difference set operation; and-   U_(x) is a set containing subbands in the full set Ω that are not in    set F_(x).

The three sectors in each 3-sector cell may be assigned a different pairof usable set and forbidden set. For example, sector 1 may be assignedusable set U1 and forbidden set F1, sector 2 may be assigned usable setU2 and forbidden set F2, and sector 3 may be assigned usable set U3 andforbidden set F3. Each sector is also aware of the forbidden setsassigned to neighboring sectors. Thus, sector 1 is aware of forbiddensets F2 and F3 assigned to neighboring sectors 2 and 3, sector 2 isaware of forbidden sets F1 and F3 assigned to neighboring sectors 1 and3, and sector 3 is aware of forbidden sets F1 and F2 assigned toneighboring sectors 1 and 2.

FIG. 5A shows a Venn diagram for the usable set U1 assigned to sector 1.Usable set U1 (shown by diagonal hashing) includes all of the N totalsubbands except for those in the forbidden set F1.

FIG. 5B shows a Venn diagram for a restricted usable set U1-2 (shown bycross-hashing) for sector 1. Restricted set U1-2 contains subbandsincluded in both the usable set U1 for sector 1 and the forbidden set F2for sector 2. Since the subbands in forbidden set F2 are not used bysector 2, the subbands in restricted set U1-2 are free of interferencefrom sector 2.

FIG. 5C shows a Venn diagram for a restricted usable set U1-3 (shown byvertical hashing) for sector 1. Restricted set U1-3 contains subbandsincluded in both the usable set U1 for sector 1 and the forbidden set F3for sector 3. Since the subbands in forbidden set F3 are not used bysector 3, the subbands in restricted set U1-3 are free of interferencefrom sector 3.

FIG. 5D shows a Venn diagram for a more restricted usable set U1-23(shown by solid fill) for sector 1. Restricted set U1-23 containssubbands included in all three of the usable set U1 for sector 1, theforbidden set F2 for sector 2, and the forbidden set F3 for sector 3.Since the subbands in forbidden sets F2 and F3 are not used by sectors 2and 3, respectively, the subbands in restricted set U1-23 are free ofinterference from both sectors 2 and 3.

As shown in FIGS. 5A through 5D, the restricted usable sets U1-2, U1-3and U1-23 are different subsets of the unrestricted usable set U1assigned to sector 1. Restricted usable sets U2-1, U2-3 and U2-13 may beformed for sector 2, and restricted usable sets U3-1, U3-2 and U3-12 maybe formed for sector 3 in similar manner. Table 1 lists the varioususable subband sets for the three sectors and the manner in which thesesets may be formed. The “reuse” sets in Table 1 are described below.

TABLE 1 Reuse Set Usable Subband Sets Description (1) U₁ = Ω \ F₁Main/unrestricted usable set for sector 1 (1, 2) U_(1–2) = U₁ ∩ F₂ = F₂\ Restricted usable set with no (F₁ ∩ F₂) interference from sector 2 (1,3) U_(1–3) = U₁ ∩ F₃ = F₃ \ Restricted usable set with no (F₁ ∩ F₃)interference from sector 3 (1, 2, U_(1–23) = U₁ ∩ F₂ ∩ F₃ = Morerestricted usable set with no 3) F₂ ∩ F₃ interference from sectors 2 & 3(2) U₂ = Ω \ F₂ Main/unrestricted usable set for sector 2 (2, 1) U_(2–1)= U₂ ∩ F₁ = F₁ \ Restricted usable set with no (F₁ ∩ F₂) interferencefrom sector 1 (2, 3) U_(2–3) = U₂ ∩ F₃ = F₃ \ Restricted usable set withno (F₂ ∩ F₃) interference from sector 3 (2, 1, U_(2–13) = U₂ ∩ F₁ ∩ F₃ =More restricted usable set with no 3) F₁ ∩ F₃ interference from sectors1 & 3 (3) U₃ = Ω \ F₃ Main/unrestricted usable set for sector 3 (3, 1)U_(3–1) = U₃ ∩ F₁ = F₁ \ Restricted usable set with no (F₁ ∩ F₃)interference from sector 1 (3, 2) U_(3–2) = U₃ ∩ F₂ = F₂ \ Restrictedusable set with no (F₂ ∩ F₃) interference from sector 2 (3, 1, U_(3–12)= U₃ ∩ F₁ ∩ F₂ = More restricted usable set with no 2) F₁ ∩ F₂interference from sectors 1 & 2

Each sector x (where x=1, 2, or 3) may allocate subbands in its usableset U_(x) to users in the sector by taking into account the users'channel conditions so that reasonably good performance may be achievedfor all users. Sector x may have weak users as well as strong users. Astrong user has a relatively good signal quality metric for its servingbase station and is typically more tolerant to higher level ofinter-sector interference. A weak user is less tolerant to inter-sectorinterference. Sector x may allocate any of the subbands in its usableset U_(x) to the strong users in the sector. Sector x may allocatesubbands in the restricted sets to the weak users in the sector. Theweak users are, in effect, restricted to certain subbands known to befree of interference from strong interfering sectors.

For example, a given user u in sector x may be allocated subbands fromusable set U_(x) for sector x. If user u is deemed to beobserving/causing high inter-sector interference from/to sector y, wherey≠x, then user u may be allocated subbands from the restricted setU_(x-y)=U_(x)∩F_(y). If user u is further deemed to be observing/causinghigh inter-sector interference from/to sector z, where z≠x and z≠y, thenuser u may be allocated subbands from the more restricted setU_(x-yz)=U_(x)∩F_(y)∩F_(z).

FIG. 6 shows an example for forming the three forbidden subband sets F1,F2 and F3. In this example, the N total subbands are partitioned into Qgroups, with each group containing 3•L subbands that are given indicesof 1 through 3L, where Q≧1 and L>1. Forbidden set F1 contains subbands1, L+1, and 2L+1 in each group. Forbidden set F2 contains subbands 1,L+2, and 2L+2 in each group. Forbidden set F3 contains subbands 2, L+1,and 2L+2 in each group. Set F12 then contains subband 1 in each group,set F13 contains subband L+1 in each group, and set F23 contains subband2L+2 in each group.

In general, each forbidden set may contain any number of subbands andany one of the N total subbands, subject to the constraints shown inequation (1) and possibly (2). To obtain frequency diversity, eachforbidden set may contain subbands taken from across the N totalsubbands. The subbands in each forbidden set may be distributed acrossthe N total subbands based on a predetermined pattern, as shown in FIG.6. Alternatively, the subbands in each forbidden set may bepseudo-randomly distributed across the N total subbands. The threeforbidden sets F1, F2 and F3 may also be defined with any amount ofoverlap. The amount of overlap may be dependent on various factors suchas, for example, the desired effective reuse factor for each sector(described below), the expected number of weak users in each sector, andso on. The three forbidden sets may overlap each other by the sameamount, as shown in FIG. 4, or by different amounts.

Each user may be associated with a “reuse” set that contains the servingsector for the user as well as strong interferers/ees, if any, for theuser. The serving sector is denoted by boldfaced and underlined text inthe reuse set. The strong interferers/ees are denoted by normal text,after the boldfaced and underlined text for the serving sector, in thereuse set. For example, a reuse set of (2, 1, 3) denotes sector 2 beingthe serving sector and sectors 1 and 3 being strong interferers/ees.

Strong interferers to a given user u on the forward link are typicallyfixed and may be specifically identified, e.g., based on pilotstransmitted by the sectors. Strong interferers to user u on the reverselink may not be easily identified by forward link measurement performedby user u and may be deduced, e.g., based on reverse link interferencemeasurement by the serving base station of user u. Strong interfereesfor user u may also be specifically identified or deduced. Stronginterferers/ees for each user may be determined in various manners.

In one embodiment, strong interferers/ees for a given user u aredetermined based on received pilot powers, as measured by user u, fordifferent sectors. Each sector may transmit a pilot on the forward linkfor various purposes such as signal detection, timing and frequencysynchronization, channel estimation, and so on. User u may search forpilots transmitted by the sectors and measure the received power of eachdetected pilot. User u may then compare the received pilot power foreach detected sector against a power threshold and add the sector to itsreuse set if the received pilot power for the sector exceeds the powerthreshold.

In another embodiment, strong interferers/ees for user u are determinedbased on an “active” set maintained by user u. The active set containsall sectors that are candidates for serving user u. A sector may beadded to the active set, e.g., if the received pilot power for thesector, as measured by user u, exceeds an add threshold (which may ormay not be equal to the power threshold described above). Each user inthe system may be required to (e.g., periodically) update its active setand to report the active set to its serving sector. The active setinformation may be readily available at the sector and may be used forrestrictive reuse.

In yet another embodiment, strong interferers/ees for user u aredetermined based on received pilot powers, as measured at differentsectors, for user u. Each user may also transmit a pilot on the reverselink for various purposes. Each sector may search for pilots transmittedby users in the system and measure the received power of each detectedpilot. Each sector may then compare the received pilot power for eachdetected user against the power threshold and inform the user's servingsector if the received pilot power exceeds the power threshold. Theserving sector for each user may then add sectors that have reportedhigh received pilot powers to that user's reuse set.

In yet another embodiment, strong interferers/ees for user u aredetermined based on a position estimate for user u. The position of useru may be estimated for various reasons (e.g., to provide locationservice to user u) and using various position determination techniques(e.g., Global Positioning System (GPS), Advanced Forward LinkTrilateration (A-FLT), and so on, which are known in the art). Thestrong interferers/ees for user u may then be determined based on theposition estimate for user u and sector/cell layout information.

Several embodiments for determining strong interferers/ees for each userhave been described above. Strong interferers/ees may also be determinedin other manners and/or based on other quantities besides received pilotpower. A good signal quality metric for determining strong interfererson the forward link is an average SINR measured at a user for a basestation, which is also called “geometry”. A good signal quality metricfor determining strong interferees on the reverse link is a channel gainmeasured at a user for a base station, since SINR measurement is notavailable at the user for the base station. A single reuse set may bemaintained for both the forward and reverse links, or separate sets maybe used for the two links. The same or different signal quality metricsmay be used to update the sectors in the reuse set for the forward andreverse links.

In general, strong interferers/ees may be specifically identified basedon direct measurements (e.g., for the forward link) or deduced based onrelated measurements, sector/cell layout, and/or other information(e.g., for the reverse link). For simplicity, the following descriptionassumes that each user is associated with a single reuse set thatcontains the serving sector and other sectors (if any) deemed to bestrong interferers/ees for the user.

In a well-designed system, a weak user should have a relatively fairsignal quality metric for at least one neighboring sector. This allowsthe weak user to be handed off from a current serving sector to aneighboring sector if necessary. Each such neighboring sector may bedeemed as a strong interferer/ee to the weak user and may be included inthe user's reuse set.

FIG. 7A shows an example distribution of four users in a cluster ofseven sectors. In this example, user 1 is located near the middle ofsector 1 and has a reuse set of (1). User 2 is located near the boundarybetween sectors 1 and 3 and has a reuse set of (1, 3). User 3 is alsolocated near the boundary between sectors 1 and 3 but has a reuse set of(3, 1). User 4 is located near the boundary of sectors 1, 2 and 3 andhas a reuse set of (1, 2, 3).

FIG. 7B shows a non-interference pattern for user 1 in FIG. 7A. User 1is allocated subbands in usable set U1 since its reuse set is (1).Because users in sector 1 are allocated orthogonal subbands, user 1 doesnot interfere with other users in sector 1. However, usable set U1 isnot orthogonal to usable sets U2 and U3 for sectors 2 and 3,respectively. Thus, user 1 observes interference from the sixneighboring sectors 2 and 3 in the first tier around sector 1. User 1typically observes interference from distant or weak interferers inthese six neighboring sectors because strong interferers (to sector1/user 1) in these neighboring sectors are allocated subbands (e.g., inrestricted sets U2-1 and U3-1) that are orthogonal to those in usableset U1. The area where other users do not interfere with user 1 is shownby cross-hashing and covers sector 1 and the edges of other sectors thatneighbor sector 1 (since the users in these neighboring sectors 2 and 3may be assigned subbands that are not used by sector 1).

FIG. 7C shows a non-interference pattern for user 2 in FIG. 7A. User 2is allocated subbands in restricted set U₁₋₃=U₁∩F₃ since its reuse setis (1, 3). Because sector 3 does not use the subbands in its forbiddenset F3, the subbands allocated to user 2 are orthogonal to the subbandsused by sector 3. Thus, user 2 does not observe any interference fromother users in sector 1 as well as users in sector 3. User 2 observesinterference from distant interferers in the three first-tierneighboring sectors 2. The area where other users do not interfere withuser 2 covers sectors 1 and 3 and the edges of sectors 2 that neighborsector 1 (for the reason noted above for FIG. 7B).

FIG. 7D shows a non-interference pattern for user 4 in FIG. 7A. User 4is allocated subbands in restricted set U₁₋₂₃=U₁∩F₂∩F₃ since its reuseset is (1, 2, 3). Because sectors 2 and 3 do not use the subbands intheir forbidden sets F2 and F3, respectively, the subbands allocated touser 4 are orthogonal to the subbands used by sectors 2 and 3. Thus,user 4 does not observe any interference from other users in sector 1 aswell as users in the six first-tier neighboring sectors 2 and 3. Thearea where other users do not interfere with user 4 covers sectors 1, 2and 3.

In FIG. 7A, users 2 and 3 are located in close proximity and would haveinterfered strongly with each other without restrictive reuse. Withrestrictive reuse, user 2 is allocated subbands in restricted setU₁₋₃=U₁∩F₃ since its reuse set is (1, 3), and user 3 is allocatedsubbands in restricted set U₃₋₁=U₃∩F₁ since its reuse set is (3, 1).Restricted sets U1-3 and U3-1 are mutually orthogonal since eachrestricted set U_(x-y) contains only subbands that are excluded from theusable set U_(y) of which the other restricted set U_(y-x) is a subset.Because users 2 and 3 are allocated subbands from orthogonal restrictedsets U1-3 and U3-1, respectively, these two users do not interfere withone another.

As shown in FIGS. 7A through 7D, the interference experienced by a userdecreases as the size of its reuse set increases. A user with a reuseset size of one (e.g., user 1 in FIG. 7B) is interfered by distantinterferers in six first-tier neighboring sectors. A user with a reuseset size of two (e.g., user 2 in FIG. 7C) is interfered by distantinterferers in three first-tier neighboring sectors. A user with a reuseset size of three is interfered by interferers in second-tier neighborsectors. In contrast, without restrictive reuse, all users in the systemwould be interfered by randomly distributed interferers from all sixfirst-tier neighboring sectors.

Restrictive reuse may be used to mitigate inter-sector interference forweak users on both the forward and reverse links. On the forward link, aweak user u in sector x may observe high inter-sector interference frombase stations for neighboring sectors that are in its reuse set. Weakuser u may be allocated subbands that are not used by these neighboringsectors and would then observe no interference from the base stationsfor these sectors. Restrictive reuse may thus directly improve the SINRsof individual weak user u.

On the reverse link, weak user u may observe high inter-sectorinterference from users in neighboring sectors that are in its reuseset. Weak user u may be allocated subbands that are not used by theseneighboring sectors and would then observe no interference from theusers in these sectors. Weak user u may also be a strong interferer tothe users in the neighboring sectors. Weak user u typically transmits ata high power level in order to improve its received SINR at its servingsector x. The high transmit power causes more interference to all usersin the neighboring sectors. By restricting weak user u to subbands notused by the neighboring sectors in the reuse set, weak user u wouldcause no interference to the users in these sectors.

When restrictive reuse is applied across the system, weak user u maybenefit from lower inter-sector interference on the reverse link even ifthe strong interferers to weak user u cannot be identified. Weak usersin neighboring sectors that have sector x in their reuse sets may bestrong interferers to weak user u as well as other users in sector x.These strong interferers may be allocated subbands that are not used bysector x and would then cause no interference to the users in sector x.User u may thus observe no inter-sector interference from these stronginterferers even though user u is not able to identify them. Restrictivereuse generally improves the SINRs of all weak users.

For both the forward and reverse links, restrictive reuse can avoid orreduce interference observed by weak users from strong interferers andthus improve the SINRs for the weak users. Restrictive reuse may reducethe variation in SINRs among users in the system. As a result, improvedcommunication coverage as well as higher overall system capacity may beachieved for the system.

FIG. 8 shows a flow diagram of a process 800 for allocating subbands tousers in a sector with restrictive reuse. Process 800 may be performedby/for each sector. Initially, strong “interference entities”, if any,for each user in the sector are identified (block 812). A stronginterference entity for a given user u may be (1) a strong interferercausing high interference to user u and/or (2) a strong interferesobserving high interference from or due to user u. A strong interferenceentity for user u may thus be (1) a base station causing highinterference to user u on the forward link, (2) another user causinghigh interference to user u on the reverse link, (3) a base stationobserving high interference from user u on the reverse link, (4) anotheruser observing high interference from user u's serving base station onthe forward link, or (5) some other entity for which mitigation ofinterference with user u is sought. The strong interference entities maybe identified based on, e.g., received pilot powers measured by the userfor different sectors, received pilot powers measured by differentsectors for the user, and so on. The strong interference entities foreach user may be included in the user's reuse set, as described above.In any case, a restricted usable set is determined for each user with atleast one strong interference entity (block 814). The restricted set foreach user may be obtained by performing an intersection set operation onthe usable set for the user's serving sector with the forbidden set foreach strong interference entity, or U_(x-y . . .) =U_(x)∩F_(y . . .) .Each user with at least one strong interference entity is allocatedsubbands in the restricted set determined for that user (block 816).Each user without a strong interference entity is allocated remainingsubbands in the usable set for the sector (block 818). The process thenterminates.

Process 800 shows allocation of subbands to weak users with at least onestrong interference entity first, then allocation of remaining subbandsto strong users. In general, the weak and strong users may be allocatedsubbands in any order. For example, users may be allocated subbandsbased on their priority, which may be determined from various factorssuch as the SINRs achieved by the users, the data rates supported by theusers, the payload size, the type of data to be sent, the amount ofdelay already experienced by the users, outage probability, the maximumavailable transmit power, the type of data services being offered, andso on. These various factors may be given appropriate weights and usedto prioritize the users. The users may then be allocated subbands basedon their priority.

Process 800 may be performed by each sector in each scheduling interval,which may be a predetermined time interval. Each sector may sendsignaling (e.g., to all users or to only users allocated differentsubbands) to indicate the subbands allocated to each user. Process 800may also be performed (1) whenever there is a change in users in thesector (e.g., if a new user is added or a current user is removed), (2)whenever the channel conditions for the users change (e.g., whenever thereuse set for a user changes), or (3) at any time and/or due to anytriggering criterion. At any given moment, all of the subbands may notbe available for scheduling, e.g., some subbands may already be in usefor retransmissions or some other purposes.

The forbidden sets represent overhead for supporting restrictive reuse.Since the subbands in forbidden set F_(x) are not used by sector x, thepercentage of the total subbands usable by sector x, which is also theeffective reuse factor for sector x, may be given as:|U_(x)|/|Ω|=(|Ω|−|F_(x)|)/|Ω|, where |U_(x)| denotes the size of setU_(x). To reduce the amount of overhead for restrictive reuse, theforbidden sets may be defined to be as small as possible. However, thesizes of the restricted sets are dependent on the sizes of the forbiddensets. Thus, the forbidden sets may be defined based on expectedrequirements for weak users and possibly other factors.

The usable and forbidden sets may be defined in various manners. In oneembodiment, the usable and forbidden sets are defined based on globalfrequency planning for the system and remain static. Each sector isassigned a usable set and a forbidden set, forms its restricted sets asdescribed above, and thereafter uses the usable and restricted sets.This embodiment simplifies implementation for restrictive reuse sinceeach sector can act autonomously, and no signaling between neighboringsectors is required. In a second embodiment, the usable and forbiddensets may be dynamically defined based on sector loading and possiblyother factors. For example, the forbidden set for each sector may bedependent on the number of weak users in neighboring sectors, which maychange over time. A designated sector or a system entity (e.g., systemcontroller 130) may receive loading information for various sectors,define the usable and forbidden sets, and assign the sets to thesectors. This embodiment may allow for better utilization of systemresources based on the distribution of users. In yet another embodiment,the sectors may send inter-sector messages to negotiate the usable andforbidden sets.

Restrictive reuse can support handoff, which refers to the transfer of auser from a current serving base station to another base station that isdeemed better. Handoff may be performed as needed to maintain goodchannel conditions for users on the edge of sector coverage (or“sector-edge” users). Some conventional systems (e.g., a Time DivisionMultiple Access (TDMA) system) support “hard” handoff whereby a userfirst breaks away from the current serving base station and thenswitches to a new serving base station. A Code Division Multiple Access(CDMA) system supports “soft” and “softer” handoffs, which allow a userto simultaneously communicate with multiple cells (for soft handoff) ormultiple sectors (for softer handoff). Soft and softer handoffs canprovide additional mitigation against fast fading.

Restrictive reuse can reduce interference for sector-edge users, whichare good candidates for handoff. Restrictive reuse can also supporthard, soft, and softer handoffs. A sector-edge user u in sector x may beallocated subbands in the restricted set U_(x-y), which is free ofinterference from neighboring sector y. Sector-edge user u may alsocommunicate with sector y via subbands in the restricted set U_(y-x),which is free of interference from sector x. Since the restricted setsU_(x-y) and U_(y-x) are disjoint, user u may simultaneously communicatewith both sectors x and y (and with no interference from stronginterferers in both sectors) for soft or softer handoff. User u may alsoperform hard handoff from sector x to sector y. Since restricted setsU_(x-y) and U_(y-x) are absent of strong interferers from sectors y andx, respectively, the received SINR of user u may not change quite asabruptly when user u is handed off from sector x to sector y, which canensure a smooth handoff.

Power control may or may not be used in combination with restrictivereuse. Power control adjusts the transmit power for a data transmissionsuch that the received SINR for the transmission is maintained at atarget SINR, which may in turn be adjusted to achieve a particular levelof performance, e.g., 1% packet error rate (PER). Power control may beused to adjust the amount of transmit power used for a given data rate,so that interference is minimized. Power control be used for certain(e.g., fixed rate) transmissions and omitted for other (e.g., variablerate) transmissions. Full transmit power may be used for a variable ratetransmission (such as a hybrid automatic retransmission (H-ARQ), whichis continual transmission of additional redundancy information for eachpacket until the packet is decoded correctly) in order to achieve thehighest rate possible for a given channel condition.

In the above embodiment for restrictive reuse, each sector is associatedwith one usable set and one forbidden set. Some other embodiments ofrestrictive reuse are described below.

In another embodiment of restrictive reuse, each sector x is assigned anunrestricted usable subband set Ux and a “limited use” subband set Lx.The unrestricted usable set contains subbands that may be allocated toany users in the sector. The limited use set contains subbands havingcertain use restrictions such as, e.g., a lower transmit power limit.Sets Ux and Lx may be formed in the manner described above for sets Uxand Fx, respectively.

Each sector x may allocate the subbands in sets Ux and Lx by taking intoaccount the channel conditions for the users so that good performancemay be achieved for all users. The subbands in set Ux may be allocatedto any user in sector x. Weak users in sector x may be allocatedsubbands in (1) a restricted set U_(x-y)=U_(x)∩L_(y), if highinterference is observed from neighboring sector y, (2) a restricted setU_(x-z)=U_(x)∩L_(x), if high interference is observed from neighboringsector z, or (3) a restricted set U_(x-yz)=U_(x)∩L_(y)∩L_(z), if highinterference is observed from neighboring sectors y and z. Strong usersin sector x may be allocated subbands in Lx.

A strong user v in sector x has a good signal quality metric for itsserving sector x and may be allocated subbands in the limited use setLx. On the forward link, sector x may transmit at or below the lowerpower limit for set Lx to strong user v. On the reverse link, stronguser v may transmit at or below the lower power limit to serving sectorx. Good performance may be achieved for strong user v for both theforward and reverse links, even with the lower transmit power, becauseof the good signal quality metric achieved by strong user v for sectorx.

Strong user v typically has poor signal quality metrics for neighboringsectors. On the forward link, the lower transmit power used by sector xfor strong user v causes low (and typically tolerable) levels ofinterference to users in neighboring sectors. On the reverse link, thelower transmit power used by strong user v plus the lower channel gainsfor neighboring sectors result in low (and typically tolerable) levelsof interference to the users in the neighboring sectors.

In yet another embodiment of restrictive reuse, each reuse set isassociated with a sorted list of subband sets that may be used for thereuse set. Due to frequency planning restrictions, the bandwidth of somerestricted sets may be quite small, such as restricted set U1-23 whichcorresponds to reuse set (1,2,3). Suppose user u observes highinterference from sectors 2 and 3 and is assigned to reuse set (1,2,3).Although user u will experience higher SINR due to reduced interference,the bandwidth loss resulting from a restriction to a small restrictedset U1-23 may be detrimental in terms of the achievable throughput ofuser u. Hence, for users in reuse set (1,2,3), a sorted list of subbandsets with descending preference may be defined, e.g., (U1-23, [U1-2,U1-3], U1), where the subband sets within the square brackets have equalpreference. The users in reuse set (1,2,3) may then use largerbandwidth, if necessary, by using additional subband sets in the sortedlist associated with reuse set (1,2,3). For users in reuse set (1,2),the sorted list may be (U1-2, U1, U1-3, U1-23). For users in reuse set(1), the sorted list may be (U1, [U1-2, U1-3], U1-23). The sorted listfor each reuse set may be defined to (1) reduce the amount ofinterference observed by the users in the reuse set and/or (2) reducethe amount of interference caused by the users in the reuse set.

In still yet another embodiment of restrictive reuse, each sector x isassigned multiple (M) usable sets and multiple (e.g., M) forbidden sets.The number of usable sets may or may not be equal to the number offorbidden sets. As an example, multiple (M) pairs of usable andforbidden sets may be formed, with the usable set Ux and the forbiddenset Fx in each pair being formed such that each of the N total subbandsis included in only set Ux or set Fx, e.g., Ω=U_(x)∪F_(x), where “∪”denotes a union set operation. However, in general, the M usable setsand M forbidden sets may be formed in various manners.

For example, the M usable sets may be formed such that they aresuccessively smaller subsets of the largest usable set. Each sector maythen use the smallest possible usable set based on its loading. This mayreduce the total interference to neighboring sectors when the sector ispartially loaded. This may also increase the variation in theinterference observed by neighboring sectors, which may be exploited toimprove overall system performance.

The M forbidden sets may be formed such that they are non-overlapping.The number of weaker users in each sector and their data requirementsare typically not known a priori. Each sector may utilize as manyforbidden sets for neighboring sectors as required to support its weakusers. For example, sector x may utilize subbands in more forbidden setsfor sector y to provide higher data rates to one or more weak users insector x observing high interference from sector y, or to support moreof these weak users. The sectors may coordinate usage of the forbiddensets.

In general, each sector may be assigned any number of unrestrictedusable subband sets and any number of “constrained” subband sets. Aconstrained subband set may be a forbidden subband set or a limited usesubband set. As an example, a sector may be assigned multipleconstrained subband sets. One constrained subband set may be a forbiddensubband set, and the remaining constrained subband set(s) may havedifferent transmit power limits and may be allocated to different tiersof strong users. As another example, a sector may be assigned multipleconstrained subband sets, where each constrained subband set may have adifferent transmit power limit (i.e., no forbidden set). The use ofmultiple usable and/or constrained sets for each sector may allow forbetter matching of subbands to weak users in different sectors.

For clarity, restrictive reuse has been specifically described for asystem with 3-sector cells. In general, restrictive reuse may be usedwith any reuse pattern. For a K-sector/cell reuse pattern, the forbiddenset for each sector/cell may be defined such that it overlaps with theforbidden set for each of the other K−1 sectors/cells, and may overlapwith different combinations of other forbidden sets. Each sector/cellmay form different restricted sets for different neighboring sectorsbased on its usable set and the forbidden sets for the neighboringsectors. Each sector/cell may then use the usable and restricted sets asdescribed above.

Restrictive reuse has also been described for an OFDMA system.Restrictive reuse may also be used for a TDMA system, a FrequencyDivision Multiple Access (FDMA) system, a CDMA system, a multi-carrierCDMA system, an Orthogonal Frequency Division Multiple Access (OFDMA)system, and so on. A TDMA system uses time division multiplexing (TDM),and transmissions for different users are orthogonalized by transmittingin different time intervals. An FDMA system uses frequency divisionmultiplexing (FDM), and transmissions for different users areorthogonalized by transmitting in different frequency channels orsubbands. In general, the system resources to be reused (e.g., frequencysubbands/channels, time slots, and so on) may be partitioned into usableand forbidden sets. The forbidden sets for neighboring sectors/cellsoverlap one another, as described above. Each sector may form restrictedsets based on its usable set and the forbidden sets for neighboringsectors/cells, as described above.

Restrictive reuse may be used for a Global System for MobileCommunications (GSM) system. A GSM system may operate in one or morefrequency bands. Each frequency band covers a specific range offrequencies and is divided into a number of 200 kHz radio frequency (RF)channels. Each RF channel is identified by a specific ARFCN (absoluteradio frequency channel number). For example, the GSM 900 frequency bandcovers ARFCNs 1 through 124, the GSM 1800 frequency band covers ARFCNs512 through 885, and the GSM 1900 frequency band covers ARFCNs 512through 810. Conventionally, each GSM cell is assigned a set of RFchannels and only transmits on the assigned RF channels. To reduceinter-cell interference, GSM cells located near each other are normallyassigned different sets of RF channels such that the transmissions forneighboring cells do not interfere with one another. GSM typicallyemploys a reuse factor greater than one (e.g., K=7).

Restrictive reuse may be used to improve efficiency and reduceinter-sector interference for a GSM system. The available RF channelsfor the GSM system may be used to form K pairs of usable and forbiddensets (e.g., K=7), and each GSM cell may be assigned one of the K setpairs. Each GSM cell may then allocate RF channels in its usable set tousers in the cell and RF channels in its restricted sets to weak users.Restrictive reuse allows each GSM cell to use a larger percentage of theavailable RF channels, and a reuse factor closer to one may be achieved.

Restrictive reuse may also be used for a multi-carrier communicationsystem that utilizes multiple “carriers” for data transmission. Eachcarrier is a sinusoidal signal that may be independently modulated withdata and is associated with a particular bandwidth. One such system is amulti-carrier IS-856 system (also called 3×-DO (data-only)) that hasmultiple 1.23 MHz carriers. Each sector/cell in the system may beallowed to use all carriers or only a subset of the carriers. Asector/cell may be forbidden to use a given carrier to avoid causinginterference on the carrier, which may allow other sectors/cells usingthis carrier to observe less (or no) interference, achieve higher SINR,and attain better performance. Alternatively, a sector/cell may beconstrained to use a lower transmit power limit on a given carrier toreduce interference on the carrier. For each sector, the constrained(forbidden or limited use) carrier(s) may be statically or dynamicallyassigned.

Each sector may assign its users to its usable carrier(s). Each sectormay also assign each user to a carrier in a manner to avoid stronginterferers/ees for the user. For example, if multiple usable carriersare available, then a user may be assigned one of the carriers havingless interference for the user (e.g., a carrier not used by a stronginterferer to the user).

The processing for data transmission and reception with restrictivereuse is dependent on system design. For clarity, exemplary transmittingand receiving entities in a frequency hopping OFDMA system for therestrictive reuse embodiment with a pair of usable and forbidden subbandsets for each sector are described below.

FIG. 9 shows a block diagram of an embodiment of a transmitting entity110 x, which may be the transmit portion of a base station or aterminal. Within transmitting entity 110 x, an encoder/modulator 914receives traffic/packet data from a data source 912 for a given user u,processes (e.g., encodes, interleaves, and modulates) the data based ona coding and modulation scheme selected for user u, and provides datasymbols, which are modulation symbols for data. Each modulation symbolis a complex value for a point in a signal constellation for theselected modulation scheme. A symbol-to-subband mapping unit 916provides the data symbols for user u onto the proper subbands determinedby an FH control, which is generated by an FH generator 940 based on thetraffic channel assigned to user u. FH generator 940 may be implementedwith look-up tables, pseudo-random number (PN) generators, and so on.Mapping unit 916 also provides pilot symbols on subbands used for pilottransmission and a signal value of zero for each subband not used forpilot or data transmission. For each OFDM symbol period, mapping unit916 provides N transmit symbols for the N total subbands, where eachtransmit symbol may be a data symbol, a pilot symbol, or a zero-signalvalue.

An OFDM modulator 920 receives N transmit symbols for each OFDM symbolperiod and generates a corresponding OFDM symbol. OFDM modulator 920typically includes an inverse fast Fourier transform (IFFT) unit and acyclic prefix generator. For each OFDM symbol period, the IFFT unittransforms the N transmit symbols to the time domain using an N-pointinverse FFT to obtain a “transformed” symbol that contains N time-domainchips. Each chip is a complex value to be transmitted in one chipperiod. The cyclic prefix generator then repeats a portion of eachtransformed symbol to form an OFDM symbol that contains N+C chips, whereC is the number of chips being repeated. The repeated portion is oftencalled a cyclic prefix and is used to combat inter-symbol interference(ISI) caused by frequency selective fading. An OFDM symbol periodcorresponds to the duration of one OFDM symbol, which is N+C chipperiods. OFDM modulator 920 provides a stream of OFDM symbols. Atransmitter unit (TMTR) 922 processes (e.g., converts to analog,filters, amplifies, and frequency upconverts) the OFDM symbol stream togenerate a modulated signal, which is transmitted from an antenna 924.

Controller 930 directs the operation at transmitting entity 110 x.Memory unit 932 provides storage for program codes and data used bycontroller 930.

FIG. 10 shows a block diagram of an embodiment of a receiving entity 120x, which may be the receive portion of a base station or a terminal. Oneor more modulated signals transmitted by one or more transmittingentities are received by an antenna 1012, and the received signal isprovided to and processed by a receiver unit (RCVR) 1014 to obtainsamples. The set of samples for one OFDM symbol period represents onereceived OFDM symbol. An OFDM demodulator (Demod) 1016 processes thesamples and provides received symbols, which are noisy estimates of thetransmit symbols sent by the transmitting entities. OFDM demodulator1016 typically includes a cyclic prefix removal unit and an FFT unit.The cyclic prefix removal unit removes the cyclic prefix in eachreceived OFDM symbol to obtain a received transformed symbol. The FFTunit transforms each received transformed symbol to the frequency domainwith an N-point FFT to obtain N received symbols for the N subbands. Asubband-to-symbol demapping unit 1018 obtains the N received symbols foreach OFDM symbol period and provides received symbols for the subbandsassigned to user u. These subbands are determined by an FH controlgenerated by an FH generator 1040 based on the traffic channel assignedto user u. A demodulator/decoder 1020 processes (e.g., demodulates,deinterleaves, and decodes) the received symbols for user u and providesdecoded data to a data sink 1022 for storage.

A controller 1030 directs the operation at receiving entity 120 x. Amemory unit 1032 provides storage for program codes and data used bycontroller 1030.

For restrictive reuse, each sector (or a scheduler in the system)selects users for data transmission, identifies the stronginterferers/ees for the selected users, determines the usable orrestricted set for each selected user based on its stronginterferers/ees (if any), and allocates subbands (or assigns trafficchannels) from the proper sets to the selected users. Each sector thenprovides each user with its assigned traffic channel, e.g., viaover-the-air signaling. The transmitting and receiving entities for eachuser then perform the appropriate processing to transmit and receivedata on the subbands indicated by the assigned traffic channel.

The restrictive reuse techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the processing units used to identify stronginterferers/ees, determine restricted sets, allocate subbands, processdata for transmission or reception, and perform other functions relatedto restrictive reuse may be implemented within one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, the restrictive reuse techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory unit 932 in FIG. 9 or memory unit 1032 inFIG. 10) and executed by a processor (e.g., controller 930 in FIG. 9 or1030 in FIG. 10). The memory unit may be implemented within theprocessor or external to the processor.

Restrictive Reuse Approach and Schedulers

Restrictive reuse approach reduces inter-cell interference in an OFDMAbased system. The key idea behind restrictive is to intelligently deployfrequency reuse for selected users based their channel qualities. InCDMA systems, active set has been defined for each user for handoffpurposes. Sectors in the active set of a user usually contribute mostinterference to this user's reception on the FL and being interferedmost severely by this user's transmission on RL. Avoiding interferencefrom sectors in a user's active set is expected to reduce theinterference on both FL and RL.

Schedulers in wireless networks need to be modified to take advantage ofthe SINR improvement through restrictive reuse. For voice traffic, thevoice capacity is usually limited by the SINR of the worst users in anetwork. Since a voice user will occupy some narrow bandwidth for arelatively long duration, there is a direct capacity improvement byassigning a static frequency reuse set to the user to improve the userSINR throughout the duration of a call. For a user with bursty traffic,tradeoff needs to be made among frequency sets that have different SINR,available bandwidth and offered load (from other users on this reuseset). The scheduler is further complicated if fairness criteria such asequal grade of service (EGoS) or proportional fairness need to beenforced for users from different reuse set.

An EGoS restrictive reuse set management can be used in conjunction with3G schedulers to ensure EGoS on FL transmissions. This scheduler willassign users to static frequency reuse set then let the 3G scheduler toschedule packet transmissions over each reuse set.

Restrictive reuse is a global frequency planning scheme that takes intoaccount the channel and interference measured by users. In the followingembodiment, each sector is assigned a color of 0, 1 or 2. The overallbandwidth available in a network is divided into 7 frequency sets withuniversal reuse, ⅓ reuse and ⅔ reuses. Each reuse frequency set is thenlabeled with a 3-bit binary mask, where a “1” at the ith positionindicates that it is used by sectors of color i. For example, 110indicates a ⅔ frequency reuse set that is used by sectors of color 0 and1 but not sectors of color 2; 100 indicates a ⅓ frequency reuse set thatis used by sectors of color 0 but not sectors of color 1 and 2. Thelabels of frequency sets {U0, U1, U2, U3, U4, U5, U6} are given by {111,110, 101, 011, 100, 010, 001}. With frequency planning, users may avoiddominant interferers by using a ⅓ or ⅔ reuse frequency set.

In 3G networks, the fairness among data users is enforced by thescheduler. In a network where the forward link transmissions to usersare time multiplexed, the user with the highest scheduling metric willbe scheduled for transmission over the scheduling time slot. Thescheduling metric is usually computed based not only on the fairnessmetric but also the channel desirability to take advantage of themulti-user diversity (MUD). Let λn denote the throughput of user n overa specified window. Let μ_(n) and μ _(n) denote the instant and averagespectral efficiency of user n. The fairness metric F_(n) is given by

$\begin{matrix}{{F_{n} = \frac{1}{\lambda_{n}}},} & {{Eq}\mspace{20mu}(4)}\end{matrix}$

for an EGoS scheduler and

$\begin{matrix}{{F_{n} = \frac{\mu_{n}}{\lambda_{n}}},} & {{Eq}\mspace{20mu}(5)}\end{matrix}$

for a proportional fair scheduler.

The channel desirability metric is given by

$\begin{matrix}{T_{n} = {\frac{\mu_{n}}{\mu_{n}}.}} & {{Eq}\mspace{20mu}(6)}\end{matrix}$

The scheduling metric could be calculated as the product of the fairnessmetric and the channel desirability metric as given byS_(n)=F_(n)T_(n).  Eq (7)Restrictive Reuse Set Management

Assign users to restrictive reuse sets such that the sector throughputis maximized with the EGoS fairness constraint in accordance with anembodiment. Restrictive reuse may cause fairness problems if the reuseset assignment is carried out based solely on user SINR characteristicswithout considering reuse set bandwidth and reuse set loading. When auser is assigned to a ⅔ or ⅓ reuse set, its interference level issignificantly reduced compared to the universal reuse set.

For EGoS fairness, intuitively it is desirable to equalize the SINRamong all users by assigning weak users to a ⅔ or ⅓ reuse sets. However,the individual user performance, such as throughput and delay, alsodepends on the bandwidth allocation to each user within a reuse set. TheEGoS fairness within a reuse set can be achieved by EGoS schedulers thatoperate independently over each reuse set. The fairness between reusesets is obtained utilizing an restrictive reuse set management approachin accordance with an embodiment.

The aggregated reuse set throughput with EGoS fairness among users canbe approximated by the product of the harmonic mean of user spectralefficiency and the bandwidth of a reuse set. Consider the scenario of Nactive users and M non-forbidden reuse sets in a sector. Let μ_(n,m)denotes the spectral efficiency of user n over reuse set m. Let S_(m),reuse sector loading, denote the set of users assigned to reuse set m,where set 0 denote the 1/1 reuse set. Let B_(m) denote the bandwidth ofreuse set m. The EGoS sector throughput can be approximated by

${B_{m}\frac{S_{m}}{\sum\limits_{i \in S_{m}}{1/\mu_{i,m}}}},$where |.| denote the size of a set. The estimated user throughput overreuse set m is then given by

$\begin{matrix}{\eta_{m}{\frac{B_{m}}{\sum\limits_{i \in S_{m}}{1/\mu_{i,m}}}.}} & {{Eq}\mspace{20mu}(8)}\end{matrix}$

In the proposed approach, reuse set memberships are allocated tomaximize the sector throughput under the constraint that the same userthroughput is achieved over all reuse sets.

For a TDMA system, the previous analysis is valid with bandwidthreplaced by time slots. For example, Eq (8) is still valid for estimateduser throughput if B_(m) denotes the time interval instead of bandwidthin a TDMA system.

For a Multi-Carrier CDMA (MC_CDMA) system, same analysis also holdswhile the granularity of reuse set bandwidth are limited to multiples ofa single carrier bandwidth.

FIG. 11 shows a flowchart of a reuse set management approach inaccordance with an embodiment

1. Determine μ_(n,m) for n=[1 . . . N] and m=[1 . . . M].

2. Initialize S₀ with the set {1 . . . N} and initialize other S_(m), swith empty sets.

3. Move the weakest M−1 connections, i.e., connections with the smallestspectral efficiency μ_(n,0), from S₀ to the other M−1 sets, where eachset has one connection only.

4. Compute the estimated user throughput over each reuse set accordingto Eq (8) assuming an independent EGoS scheduler is implemented overeach set.

5. Let m_(w) denote the index of the reuse set with the minimum userthroughput η_(m). Let n_(w) denote the index of the weakest user inreuse set m_(w). The objective of the rest of the approach is to moveuser n_(w) to a suitable reuse set, such that after the move thethroughput of user n_(w) is improved the most. Obviously, users in reuseset m_(w) will also enjoy higher throughput due to improved averagechannel quality and one less user to compete for resources in reuse setm_(w).

6. Find the reuse set m_(d) in which user n_(w) can achieve the highestthroughput. If m_(d) is the original set m_(w), no improvement ispossible by moving a user from a low throughput set to other reuse set.Exit reuse set management program.

7. Move user n_(w) from reuse set S_(m) _(w) to reuse set S_(m) _(d) .Go to step 4.

In the algorithm above, the user throughputs in all reuse sets areadequately equalized by iteratively moving users from low throughputreuse sets to high throughput reuse sets. Further enhancement of thisapproach is possible by adding one more step after step 6. Morespecifically, instead of moving one user from a low throughput reuse setto a high throughput reuse set in each iteration, two reuse sets mayswap users if the throughputs in both reuse sets are increased.

In FIG. 12, the average SINR (geometry) of an OFDMA system with andwithout reuse set management approach are illustrated. Reuse2 andResuse3 refer to Restrictive reuse approaches with maximum reuse setsize 2 and 3, respectively. With 25% partial loading and Resuse3, thegeometry distribution is uniformly shifted to the right by 3.5 dB. Themean spectral efficiency is improved by 57%, and the estimatedthroughput is improves by 18% considering 25% partial loading.

FIG. 12 shows the geometry of Reuse2 and Reuse3 with 25% partial loadingand EGoS reuse set management in accordance with an embodiment.

The restrictive reuse set management approach may be used based on longterm channel quality measurements. Although short term fading may causelarge variation in user spectral efficiency over time, the difference ofa user's spectral efficiencies between two reuse sets has been observedto be a much slower random process compared with the short term fading.The restrictive reuse set membership updating interval could be set tothe active set updating interval as in conventional CDMA systems.

In an embodiment, a separate EGoS scheduler is implemented over eachreuse set. Equal grade of service among users of the same reuse set isachieved through the EGoS approach as described by steps (1), (3) and(4) where μ_(n) replaced by μ_(n,m) for reuse set m. Equal grade ofservice among different reuse sets is achieved through the reuse setmanagement approach as described.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of allocating system resources in a wireless communicationsystem, comprising: determining dimension of each reuse set; determiningloading of each reuse set; determining spectral efficiencies of allusers sharing each reuse set; estimating an equal-grade-of-service(EGoS) user throughput over each reuse set based on the dimension ofeach reuse set, loading of each reuse set, and spectral efficiencies ofall users sharing each reuse set; and equalizing user throughputs in allreuse sets.
 2. The method of claim 1, wherein user throughputs in allreuse sets are equalized by iteratively moving users from low throughputreuse sets to high throughput reuse sets.
 3. The method of claim 2,wherein two reuse sets swap users if the throughputs in both reuse setsare increased.
 4. The method of claim 1, wherein the EGoS userthroughput, η, over reuse set m (S_(m)) is estimated by${\eta_{m}\frac{B_{m}}{\sum\limits_{i \in S_{m}}{1/\mu_{i,m}}}},$ whereB_(m) denote the dimension of reuse set m and μ_(i,m) denotes thespectral efficiency of user i over reuse set m.
 5. The method of claim1, further comprising maximizing sector throughput under the constraintof EGoS user throughput over each reuse set.
 6. The method of claim 5,wherein sector throughput is maximized by maximizing${B_{m}\frac{S_{m}}{\sum\limits_{i \in S_{m}}{1/\mu_{i,m}}}},$ whereB_(m) denote the dimension of reuse set m and μ_(i,m) denotes thespectral efficiency of user i over reuse set m (Sm).
 7. The method ofclaim 5, further comprising scheduling a user from each reuse set. 8.The method of claim 1, wherein the dimension is a subcarrier in an OFDMAsystem.
 9. The method of claim 1, wherein the dimension is atimeslot-subcarrier in an OFDMA system.
 10. The method of claim 1,wherein the dimension is a timeslot in a TDMA system.
 11. The method ofclaim 1, wherein the dimension is a carrier in an MC-CDMA system.
 12. Anapparatus operable to allocate system resources in a wirelesscommunication system, comprising: a controller operative to determiningdimension of each reuse set; determining loading of each reuse set;determining spectral efficiencies of all users sharing each reuse set;estimating an equal-grade-of-service (EGoS) user throughput over eachreuse set based on the dimension of each reuse set, loading of eachreuse set, and spectral efficiencies of all users sharing each reuseset; and equalizing user throughputs in all reuse sets.
 13. A controllerprogrammed to execute a method of estimating interference in a wirelesscommunication system, the method comprising: determining dimension ofeach reuse set; determining loading of each reuse set; determiningspectral efficiencies of all users sharing each reuse set; estimating anequal-grade-of-service (EGoS) user throughput over each reuse set basedon the dimension of each reuse set, loading of each reuse set, andspectral efficiencies of all users sharing each reuse set; andequalizing user throughputs in all reuse sets.
 14. The apparatus ofclaim 13, wherein user throughputs in all reuse sets are equalized byiteratively moving users from low throughput reuse sets to highthroughput reuse sets.
 15. The apparatus of claim 13, wherein the EGoSuser throughput, η, over reuse set m (S_(m)) is estimated by${\eta_{m} = \frac{B_{m}}{\sum\limits_{i \in S_{m}}\;{1/\mu_{i,m}}}},$where B_(m) denote the dimension of reuse set m and μ_(i,m) denotes thespectral efficiency of user i over reuse set m.
 16. The apparatus ofclaim 13, wherein the controller is further programmed to performmaximizing sector throughput under the constraint of EGoS userthroughput over each reuse set.
 17. An apparatus operable to allocatesystem resources in a wireless communication system, comprising: meansfor determining dimension of each reuse set; means for determiningloading of each reuse set; means for determining spectral efficienciesof all users sharing each reuse set; means for estimating anequal-grade-of-service (EGoS) user throughput over each reuse set basedon the dimension of each reuse set, loading of each reuse set, andspectral efficiencies of all users sharing each reuse set; and means forequalizing user throughputs in all reuse sets.
 18. The apparatus ofclaim 17, further comprising means for maximizing sector throughputunder the constraint of EGoS user throughput over each reuse set. 19.The apparatus of claim 17, wherein the means for equalizing comprisesmeans for equalizing user throughputs in all reuse sets by iterativelymoving users from low throughput reuse sets to high throughput reusesets.
 20. The apparatus of claim 17, wherein the means for estimatingEGoS user throughput, η, over reuse set m (S_(m)) comprises means forestimating by${\eta_{m} = \frac{B_{m}}{\sum\limits_{i \in S_{m}}\;{1/\mu_{i,m}}}},$where B_(m) denote the dimension of reuse set m and μ_(i,m) denotes thespectral efficiency of user i over reuse set m.
 21. The apparatus ofclaim 17, further comprising means for maximizing sector throughputunder the constraint of EGoS user throughput over each reuse set.
 22. Acomputer readable media embodying a method for allocating systemresources in a wireless communication system, comprising: determiningdimension of each reuse set; determining loading of each reuse set;determining spectral efficiencies of all users sharing each reuse set;estimating an equal-grade-of-service (EGoS) user throughput over eachreuse set based on the dimension of each reuse set, loading of eachreuse set, and spectral efficiencies of all users sharing each reuseset; and equalizing user throughputs in all reuse sets.